How does Windows choose which drives to back up?

If you use Windows Backup to back up your files and you choose the default option (where Windows chooses which files to back up), Windows will back up local data files saved in libraries, files saved on the desktop, and files saved in default Windows folders. If your files span multiple drives, they will be included in the backup as long as they are in a library or in one of the folders being backed up. You can customize what is included in your backup by choosing individual folders, libraries, and drives. Windows will only back up drives that are formatted using the NTFS file system. For more information about how Windows chooses which files to back up, see How does Windows choose which files to back up?